Hemoglobin as a prognostic marker for neurological outcomes in post-cardiac arrest patients: a meta-analysis

The aim of this study was to investigate the relationship between serum level of hemoglobin and neurological outcomes following cardiac arrest. Relevant studies were identified by searching electronic databases including PubMed, Web of Science, Cochrane Library, and Embase from June 2012 through April 2023. Articles were rigorously reviewed for their study inclusion and exclusion criteria. Pooled effect date was determined using the standardized mean difference (SMD) and 95% confidence intervals (CI). The Newcastle–Ottawa Scale was used to evaluate study quality. Subgroup analyses were conducted to determine confounding factors affecting patient outcomes. Study heterogeneity, sensitivity, and publication bias were also determined. This meta-analysis included 11 studies involving 2519 patients. Our results suggest that high serum level of hemoglobin may improve neurological prognosis(SMD = 0.60, 95%CI = 0.49–0.71, I2 = 10.85). The findings of this study indicate that serum level of hemoglobin may be associated with better neurological prognosis, perhaps an appropriate increase in serum haemoglobin levels can improve the neurological prognosis of patients in cardiac arrest.


Search strategy
All comprehensive articles published before April 2023 that estimated the neurological prognostic effect of Serum level of hemoglobin in adult patients with cardiac arrest were searched in PubMed, Web of Science, Cochrane Library, and Embase databases by two experienced reviewers (Hong-xiang Hou and Liang Zhao).The reference lists of eligible studies were also searched to identify any studies that were not identified in the initial search.
Two authors carefully reviewed the title and abstract of all articles and independently scanned all articles based on predefined inclusion and exclusion criteria.
Inclusion criteria: (1) the included population was over 18 years of age; (

Quality assessment
Two reviewers independently used the Newcastle-Ottawa Scale (NOS) for non-randomized studies to assess the quality of the included studies 10 .The NOS consists of eight items that were divided into three domains: cohort selection, comparability, and outcome assessment.Grading the quality of evidence and strength of recommendations using the GRADE (Grading of Recommendations Assessment, Development and Evaluation) method based on risk of bias, inconsistency, indirectness, imprecision, and publication bias 11,12 .At the same time, we use the Quality in Prognostic Studies (QUIPS) tool 13 to assess the risk of bias.The tool consists of six domains: study participation, study attrition, prognostic factor measurement, outcome measurement, study confounding, statistical analysis and reporting.A low, moderate or high risk of bias was assessed for each domain.The original study was reevaluated by a third author when differences arose between the primary reviewers.

Data extraction
Two independent researchers extracted the relevant data for patients from all eligible studies.Any discordant assessments were resolved by a third investigator.The extracted variables were as follows: first author ' s name, year of publication, country in which the study was conducted, geographic location, inclusion period, study type, sample size, cardiac arrest type (OHCA vs. IHCA), poor neurologic outcome (PNO), serum hemoglobin sampling time, assessment tool of outcome measurement, point of outcome measurement, mean ± standard deviation (± SD), Hb level, age, male, and CPR duration.If the mean ± SD was not available, interquartile range and median [14][15][16] were converted into mean ± SD, using the method established by Wan et al. 17 .Based on the CPC score, neurological outcomes were defined as good or poor.

Statistical analysis
The association between serum level of hemoglobin and neurological outcome was estimated for every study using the standardized mean difference (SMD) and 95% confidence interval (CI).Serum hemoglobin unit measurements differed between studies.We tried to convert it into uniform units, but we could not achieve it because only the mean value was provided in the article, so we chose to use SMD.The Cochrane Q test (p < 0.10) and the I 2 statistic were used to assess heterogeneity among the included studies.The star chart also was used to test heterogeneity.The heterogeneity results of PQ < 0.1 and/or I 2 > 50% were considered high heterogeneity using the random effects models, otherwise PQ ≥ 0.1 and/or I 2 ≤ 50% were considered as statistically significant heterogeneity using the fixed-effects model 18,19 .A funnel plot and Egger ' s linear regression test were used to assess publication bias.Sensitivity analysis was used to judge robustness.Stata version 16.0 was used to perform all analyses.

Document quality assessment and data extraction
Two studies were multi-center prospective observational studies (mPOS), and one study was a single-center prospective observational study (sPOS).The remaining eight studies were single-center retrospective observational studies (sROS).Seven studies only included OHCA patients, one study only included IHCA patients, and two studies included both OHCA and IHCA patients.One study did not clarify the CA type.All articles used CPC scores for neurologic outcomes.The baseline information of the included studies are presented in Table 1 and Supplementary Table 2.According to NOS analysis, quality scores ranged from 4 to 9, and five studies were rated as high quality.The GRADE protocol was used to assess the certainty of the evidence.The evidence for hemoglobin as a prognostic marker was rated as very low due to risk of bias or evidence of publication bias (Supplementary Fig. 11).QUIPS has been used for the evaluation of prognostic studies.The overall results of the quality assessment are shown in Supplementary Fig. 12.

Meta-analysis
The meta-analysis included 11 studies with a total of 2519 cardiac arrest patients including 647 patients with a good neurological prognosis and 1872 patients with a poor neurological prognosis.Articles in this study had heterogeneity values of I 2 = 59.7% and Q test P < 0.1, suggesting that there was heterogeneity among the literatures selected for this study.The random effect model was applied to our meta-analysis (Fig. 2).This analysis showed that serum level of hemoglobin in the good prognosis group was 0.60 higher than that in the poor prognosis group, and this difference was statistically significant (P < 0.05).A star chart was used to further investigate heterogeneity (Fig. 3).This analysis indicated the studies by Kei Hayashida et al. and Chih-Hung Wang et al. affected the heterogeneity; removing these two articles from the analysis using the random effect model improved the heterogeneity (Fig. 4).Nine studies with a total of 1598 cardiac arrest patients including a total of 518 patients with good neurological prognosis and 1080 patients with poor neurological prognosis.The SMD value of the 1589 patients was 0.6, with a 95%CI of 0.49-0.71(Z = 10.85,P < 0.05), suggesting that higher serum level of hemoglobin may be a relevant factor for better neurological prognosis.Sensitivity analysis was conducted on the remaining nine articles (Supplementary Fig. 1). Figure 5 shows that the funnel plot analysis was symmetrical with a P-value > 5 after testing for publication bias.The Begg's and Egger's tests (Supplementary Fig. 2) further suggest there was no publication bias in the nine studies.In the regression analysis (Supplementary Fig. 3), Cardiac arrest type was considered the source of heterogeneity.Therefore, subgroup analysis was conducted according to cardiac arrest type (Fig. 6).We found that cardiac arrest type affected the results of the meta-analysis.The results of

Discussion
Hemoglobin is key factor of blood oxygen-carrying capacity, and as such serum hemoglobin correlates with the prognosis of various diseases.There is accumulating evidence demonstrating that serum hemoglobin concentration is associated with poor neurological outcome in a variety of brain injuries 27 , including traumatic brain injury (TBI), subarachnoid hemorrhage (SAH), stroke, and hemorrhage [28][29][30][31] .A meta-analysis demonstrated that serum hemoglobin concentration may be associated with mortality after transcatheter aortic valve implantation 32 .
A retrospective study demonstrated that serum albumin and serum level of hemoglobin at admission predict mortality in children with TBI 33 .A previous study has shown that higher serum level of hemoglobin at admission were involved in better outcomes in patients with spontaneous, nontraumatic intracerebral hemorrhage 34 The literature indicates that risk of progression of IgA nephropathy decreases with increases in serum level of hemoglobin 35 .Further evidence suggests that a decline in serum level of hemoglobin by ≥ 3 g/dl is related to an increased risk of mortality in patients with acute coronary syndromes 36 .An earlier study demonstrated that  elevating serum hemoglobin concentration at admission may reduce the risk of death after discharge in patients with acute exacerbation of chronic obstructive pulmonary disease 37 .It has been shown that anemic mice have a worse prognosis after traumatic brain injury compared to non-anemic mice, the exact mechanism of which is unclear 38 .A meta-analysis by Mori and colleagues suggested that serum level of hemoglobin were associated with overall mortality and disease progression in patients with metastatic hormone-sensitive prostate cancer 39 .
Another meta-analysis revealed that stroke patients who present with anemia from the onset have a higher risk of mortality 40 .A recent study has shown that lower serum level of hemoglobin at admission may predict extent  www.nature.com/scientificreports/ of kidney damage in patients with type 2 diabetes 41 .To the best of our knowledge, our meta-analysis is the first to investigate the association of serum level of hemoglobin with neurological outcomes in patients following cardiac arrest.This meta-analysis showed that higher serum hemoglobin levels may improve neurological prognosis in patients who have survived after cardiac arrest.Our findings are consistent with those of Cavicchi et al. 15 .It was also reported that patients following OHCA who had higher serum level of hemoglobin achieved full neurological recovery 42 .However, because the data was incomplete in that study it was not selected for inclusion in the current meta-analysis 42 Another study reported that anemia is a risk factor for cardiac arrest, which was also a general conclusion in this meta-analysis 24 .However, Tran et al. published in 2020 that there was no significant correlation between serum level of hemoglobin and neurological prognosis of IHCA patients 43 .Our heterogeneity and subgroup analysis contradict the findings of Tran et al., in that we found that high serum level of hemoglobin can improve neurological prognosis in OHCA, IHCA and OHCA + IHCA patients.Therefore, serum hemoglobin testing may help physicians and patients choose more appropriate treatments.This meta-analysis has several limitations that should be noted.First, the number of included is small and subgroup analysis was prone to error.Second, some studies had high heterogeneity, and subgroup analysis indicated that cardiac arrest type and serum hemoglobin sampling time might be the sources of heterogeneity.All included studies were cohort studies (only two studies were multicenter studies).Thus, we need large multicentre prospective cohort type of study to confirm the results of this meta-analysis.To reduce heterogeneity, more data from other ethnic groups or countries should be included.Thirdly, the results may vary depending on the time point of CPC measurement 44 .

Conclusions
The serum hemoglobin levels of patients who have survived after cardiac arrest may be associated with a better neurological prognosis, perhaps an appropriate increase in serum hemoglobin levels can improve the neurological prognosis of patients who have survived after cardiac arrest.It remains to be seen in future studies, what are the cut-off values of the serum hemoglobin levels that recommend the initiation of transfusion therapy to increase the chances of survival with a good neurological prognosis after cardiac arrest.

Figure 1 .
Figure 1.Flow diagram of studies included in the meta-analysis.

Figure 4 .
Figure 4. Relationship between serum level of hemoglobin and neurological outcomes.favorable: good neurological prognosis group; unfavorable: poor neurological prognosis group.